# Databases > Data Warehousing - Normalization, Denormalization and Redundancy

## uzval_vee

What is the diffrence between normalization and denormalization, Does redundancy come under these topics, if so what is redundancy

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## josephreddy

normalization includes reducing redudndancy, redudndancy means data repetition.By removing the repeated data from the table we can seperate the redudndant data and create another table ,that is we are seperating repeated  data from detail table and creating a master table. like that we are sepearting the data. In this case  we are not completely removing the redudndant data from detail table we are maintaing a common column in both the tables that is in master table & detail table. that is in mastertable common column is called primary key column , detail table common column is forign key.this foreign key refers to primary key.

this is one stage normalization. normalization has 3 steps 1normalform,2,3 normalfrom.in first normal form we seperate redundant data from detail table and cerate the mastertables.
2 normalfrom we maintain data consistency by creating primarykey foreign key relations . in 3 normal from we remove any calculation columns from detail table.

this is normalization . opposite of this above normal forms  is denormalization .
that is we maintain redudndant data in deail table in that case we maintain all the transaction data in one table .that is denormalization.

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## jainbrijesh

> What is the diffrence between normalization and denormalization, Does redundancy come under these topics, if so what is redundancy


We do normalization to remove redundant data and easy managing of data.

De-normalization is done when database performance is slow.

Redundancy is the repetition of same data in different tables, for ex, if address is save in more than one table then this is redundant data.

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## sutnarcha

Normalization is the process of redefining the database structure to reduce redundancy which can also be referred as duplication of data.

There are a set of rules defined which after applying on the database structure can be referred as normal forms. Eg. The structure satisfying first rule is in first normal form and the one satisfying the second rule is in second normal form and so on.

Any deviation from the rules in the database structure is called de-normalization.

+ and  of reduced redundancy. :Big Grin:  

+ saves memory as one information is saved just once at one place
+ fastens the front-end processing in database connectivity and data retrieval.

- programmers find it difficult to query the database and relate to the requirements.
- if one information is available in just one place and that place is locked by a server, the other servers will have to be on a halt.

The disadvantages are least considered and Normalization is followed in the database design for simplicity and reduced redundency. :EEK!:

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## apurvasharma

Normalization is the process of remove redundency.
In OLTP systems, data is not so large(as compare to OLAP database).
In OLTP system, we peroform all DML operations and data retrival queries.So we normalize this database and make it robust for all query operations.
1 NF: Data should be atomic.
2 NF: All non key attributes are fully functionally dependent on key attributes.
3 NF: No transitivity exixts. X-->Y-->Z     =>     X-->  and X-->Z.

So when we normalize database, it takes very less space and perform well in much DMl operations.

In OLAP system, there is only history data exists and we only perform SELECT operation on this. So for this reason we De-normalize database for faster retrival on lagre amount of data.

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## rameshpossible

Normalization is done to remove redundancy(ie to remove duplicates)


Datawarehousing contains denormalised data purposly since its used for analysis purpose

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## sanghala

*Redundancy :* repetition of data within a database, increasing the inefficiency of the possiblity of errors in databases is called Redundancy. 

In general, databases intended for  on-line transaction processing (OLTP) are typically more normalized than databases intended for on-line analytical processing (OLAP).

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## rameshpossible

Normalisation and denormalisation are exact opposite to each other.

Normalization is done to remove redundancy(ie to remove duplicates)

The steps involved are:
1 NF (to bring data to atomic level)
2 NF (To define primary-foreign relationship)
3 NF (To remove transive dependency)

It can go upto 4NF,5NF also

Normalisation is done in OLTP system       
for transaction purpose

Now you can guess what denormalization.
It is done in OLAP system for reporting purpose(of historical data)

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